A conventional dryer felt consists of an endless conveyor belt made from a one-, two-, or three-plane fabric, wherein the various planes can be defined either by different groups of cross machine direction yarns, machine direction yarns or both. During the drying process, the upper plane, or top surface of the felt is in contact with the paper web being processed. Accordingly, it is desirable for the upper plane of the felt to have a smooth and soft surface to avoid undue marking of the finished paper. Various methods have been tried to produce a dryer felt having an upper surface which exhibits the desired smoothness and softness. While a close weave of the upper or top plane produces the desired smoothness, this advantage is offset by the comparatively high resistance of the dryer felt to the passage of water and water vapor therethrough; the material costs of a close weave felt are also quite high.
The use of soft spun yarns to replace the basic cross machine direction or filling yarns of the top plane has been tried. However, it was found that the resulting dryer felt was too unstable. Later, stuffer or center picks were added in an effort to increase the stability of dryer felts using soft yarns in the top plane. Although stability improved, the cost of producing such dryer felts was greatly increased.
Using cross machine direction or filling yarns made from a stiff core filament or fiber surrounded by a plurality of twisted filaments or fibers to replace the filling yarns of the top plane has also been tried. It has been found that, in use, the stiff core filament or fiber tends to protrude through the twisted-filament or fiber wrap and thereby causes paper marking problems.
In co-pending U.S. application Ser. No. 932,409 filed Aug. 9, 1978, of which this application is a continuation-in-part, there is disclosed an improved dryer felt which overcomes many of the short comings of the prior art. As disclosed in the co-pending application, the improved dryer felt has a soft, bulky top surface or face. A plurality of machine direction yarns and a plurality of cross machine direction yarns are interwoven in a binding relationship according to a desired weave pattern to produce the top surface. A preselected number of the yarns of the top surface are encapsulated yarns having a monofilament core encapsulated in a sheath or sleeve made from a material producing a soft, bulky texture. A sufficient number of encapsulated yarns are used to ensure that a major portion of the top surface is soft and bulky.
As disclosed in the co-pending application, the sheath defines a soft bulky outer surface which, when woven into the fabric, acts to prevent shifting or migrating of the encapsulated yarns bound into the fabric by the machine direction yarns. The monofilament core of the encapsulated yarn, may comprise either a monofilament fiber, or a bundle of fibers treated with a suitable resin such as phenolic resin, to act as a monofilament fiber.
Although a fabric made according to the teachings of the co-pending application exhibits improved strength, stability, smoothness, and softness characteristics, it has been found that these characteristics may be further improved by applying the teaching of the present invention.